A device performing such heating is sometimes called a "heater" and comprises a laminated iron magnetic circuit called the "core" which is provided with an excitation coil in order to constitute an inductor. This core may be built up from two (or more) elements or it may be constituted by a single element; FIGS. 1 to 5 show such prior art devices.
FIG. 1 is a perspective view of a first or "U-type" prior art device. It is placed at the edge of the sheet to be treated. Here, and below, reference 1 designates horizontal sheet running in the direction of arrow "V", and reference 1R designates the width of the margin to be heated.
The magnetic core of the heater comprises two half-cores: one half 52A above the sheet and another 52B below the sheet; and reference 53 designates the induction coil mounted on half-core 52A.
By virtue of these two half-cores guiding the magnetic flux created by said coil, the flux passes through the sheet across an air gap and thus induces electrical current therein which heats the sheet.
FIG. 2 is a perspective view of a second of "C-type" prior art device in which the core is referenced 52 and the induction coil 53A.
In all cases the size of the air gap must be minimized in order to maximize device efficiency. However, the value of the gap cannot be reduced to less than a limit which is defined by the greatest thickness of the thickest sheet to be treated plus a safety margin which is not less than the maximum expected vertical amplitude of flatness defects which may constitute projections from the sheet.
The "C-type" device is advantageous since it appears in practice that the flux loop can be closed with a single air gap, rather than with the two air gaps generally required for a "U-type" device.
In addition, a C-type device is suitable for an improvement which is described in French Pat. No. 85/08684 and which enables the size of the gap to be reduced: this improvement consists in hingeing the magnetic core about an axis which extends parallel to the direction of sheet movement, as shown in FIG. 3 which is an end view of a third prior art device. This figure shows two half-cores 52C and 52D excited by coils 53C and hinged about an axis 56 which is perpendicular to the plane of the figure, i.e. parallel to the direction of movement of the sheet 1. The size of the gap can be adjusted by means of an actuator 57, whereby enabling the size of the gap to be matched to different thickness of sheet to be treated. The safety margin can thus be reduced by controlling the length of the actuator using detectors which sense the tops of defects and which transmit appropriate instructions to the actuator. However, this last-mentioned possibility is not used for fear of the consequences of a failure in the detection system, which would be catastrophic for the heater device.
Persons skilled in the art nevertheless accept that heater safety is ensured if the heater can withdraw or retract readily under direct impact from a defect in the sheet.
The problem then arises of finding a disposition which enables such retraction to occur without suffering from damaging forces during impact. In practice, advantage may be drawn from the fact that defects in metal sheet give rise to dangerous projections in the upwards direction only.
A known solution in a fourth heater device of the U-type consists in providing a moving half-core 52E with a protective slope 58 (see FIG. 4 which is a partial side view) and in suspending said half-core, together with its excitation winding 53E from the end of a rocker arm 56A (see FIG. 5 which is an end view with the slope and the coil removed). This arm is supported by a shaft 56B and is provided with a counterweight 59 which is intended to minimize the force required for lifting the half-core 52E.
Such a solution using a counterweight is clearly also applicable to a C-type heater using the hinged core as shown in FIG. 3.
In practice, devices using this solution have been severely damaged by defects projecting vertically from metal sheet running at high speed.
The aim of the present invention is to reduce in simple manner the forces which need to be applied to the core of a heater device on impact between said core and a defect in the running metal sheet.